Visual acuity measures the sharpness of sight at a defined distance, typically using the Snellen eye chart. Standard healthy vision is 20/20, meaning a person can clearly see an object 20 feet away that a person with normal vision should be able to see at that same distance. Visual acuity reports like 20/200 indicate a significant degree of visual impairment, signaling that a person’s sight is substantially reduced compared to the established norm.
Understanding the Snellen Fraction
Visual acuity is reported as a fraction, such as 20/200, known as the Snellen fraction. The numerator (20) represents the distance the subject stands from the chart, while the denominator (200) indicates the distance at which a person with 20/20 vision could clearly read the same line of letters the subject can read at 20 feet.
A person with 20/200 vision must stand at 20 feet to see an object with the same clarity that a person with normal vision sees from 200 feet away. This means the visual information is ten times blurrier or requires ten times the magnification to be perceived with the same sharpness. The largest letter on the standard Snellen chart usually corresponds to this 20/200 measurement.
Visualizing the World at 20/200
Experientially, 20/200 vision creates a world that is heavily blurred and lacking in detail, as objects beyond 20 feet become indistinct. For example, a street sign readable by normal vision from a football field away requires a 20/200 person to be only 20 feet away to discern the text. Recognizing faces is a challenge, and a television screen across a living room appears as an indistinct wash of color and shape. This reduction in acuity is often accompanied by a loss of contrast sensitivity, making it harder to differentiate objects from their background, especially in low light.
Functional and Legal Implications
The measurement of 20/200 vision holds profound functional and legal significance, particularly in the United States. This level of visual acuity is the primary threshold used to define “legal blindness,” classifying a person as legally blind if their best-corrected vision is 20/200 or less in their better eye, or if their visual field is 20 degrees or less. This designation determines eligibility for specific assistance programs and government benefits. Furthermore, activities requiring fine distance vision, such as driving a motor vehicle, are prohibited in all states for individuals whose vision falls at or below this threshold.
Common Conditions Associated with 20/200 Vision
A visual acuity of 20/200 can result from several eye diseases or injuries that affect the structures responsible for sharp vision.
- Advanced age-related macular degeneration (AMD), which damages the central part of the retina. Patients with severe AMD often maintain peripheral vision but lose the central acuity necessary for reading the eye chart.
- Diabetic retinopathy, a complication of diabetes, which causes severe vision loss when blood vessels in the retina leak fluid and damage the light-sensitive tissue.
- A dense cataract, which is a clouding of the eye’s lens that obstructs the passage of light to the retina.
- Damage to the optic nerve, often from conditions like advanced glaucoma.